n, m = map(int, input().split())
inf = 10 ** 10
A = [-inf] + list(map(int, input().split())) + [inf]
B = list(map(int, input().split()))
ap = 1
lst = []
ans = 0
for b in B:
    if b > A[ap]:
        if lst:
            lst = [A[ap - 1]] + lst + [A[ap]]
            ans += A[ap] - A[ap - 1]
            ma = 0
            for i in range(len(lst) - 1):
                ma = max(ma, lst[i + 1] - lst[i])
            ans -= ma
            
        while b > A[ap]:
            ap += 1
        lst = []
    lst.append(b)

lst = [A[ap - 1]] + lst + [A[ap]]
ans += A[ap] - A[ap - 1]
ma = 0
for i in range(len(lst) - 1):
    ma = max(ma, lst[i + 1] - lst[i])
ans -= ma
print(ans)